The present invention relates to a novel process for producing a proton-conducting polymer membrane based on polyazoles, which, owing to its excellent chemical and thermal properties, can be used widely and is particularly suitable as polymer-electrolyte membrane (PEM) in PEM fuel cells.
Polyazoles such as polybenzimidazoles (®Celazole) have been known for a long time. Such polybenzimidazoles (PBI) are usually produced by reacting 3,3′,4,4′-tetraaminobiphenyl with terephthalic acid or esters thereof in the melt. The prepolymer formed solidifies in the reactor and is subsequently broken up mechanically. The pulverulent prepolymer is subsequently fully polymerized in a solid-state polymerization at temperatures of up to 400° C. to give the desired polybenzimidazoles.
To produce polymer films, the FBI is, in a further step, dissolved in polar, aprotic solvents such as dimethylacetamide (DMAc) and a film is produced by classical methods.
The basic polyazole films can subsequently be doped with concentrated phosphoric acid or sulfuric acid and then act as proton conductors and separators in polymer electrolyte membrane fuel cells (PEM fuel cells).
The acid-doped polymer membranes based on polyazoles which can be obtained in this way display an advantageous property profile. However, owing to the applications desired for PEM fuel cells, especially in the automobile sector and decentralized power and heat generation (stationary applications), these still require overall improvement.
WO 02/088219 therefore proposes the use of a proton-conducting polymer membrane based on polyazoles, which can be obtained by a process comprising the steps    A) Mixing of one or more aromatic tetraamino compounds with one or more aromatic carboxylic acids or esters thereof which comprise at least two acid groups per carboxylic acid monomer, or mixing of one or more aromatic and/or heteroaromatic diamino carboxylic acids, in polyphosphoric acid to form a solution and/or dispersion,    B) Application of a layer to a support using the mixture according to step A),    C) Heating of the sheet-like structure/layer which can be obtained according to step B) under inert gas to temperatures of up to 350° C., preferably up to 280° C., to form the polyazole polymer,    D) Treatment of the membrane formed in step C) until it is self-supporting.
However, such a procedure is relatively time-consuming and complicated. Furthermore, the polycondensation or the reaction time is not always completely reproducible, which frequently makes the production process more difficult.
Thus, for example, a reaction time of more than 35 hours is usually required for the polycondensation of 3,3′,4,4′-tetraaminobiphenyl with terephthalic acid in polyphosphoric acid. Furthermore, the precise stoichiometry of the two monomers has to be adhered to since otherwise a sufficiently high molecular weight is not built up.
A further problem is the necessity of freshly premixing the monomers. For the two monomers to become similarly well distributed in the polyphosphoric acid, they have to be premixed in powder form and added as a homogeneous mixture to the polyphosphoric acid. However, this premixing has to be carried out separately for each batch. If larger batches of monomer mixtures were to be produced beforehand, the monomers would demix during storage.
Finally, the different dissolution rates of the monomers also lead to additional problems. When the monomer mixture is stirred into and dissolved in the polyphosphoric acid, the 3,3′,4,4′-tetraaminobiphenyl goes into solution substantially more quickly than the terephthalic acid. It is frequently observed that residues of monomer powder accumulate on the stirrer or on the vessel walls and are thus withdrawn from the reaction solution. This leads to a nonstoichiometric ratio of the two monomers in the reaction mixture, which in turn has an adverse effect on the build-up of the molar mass in the polycondensation.